With a pressurized tank you will not usually get an air leak through the needle valve at full throttle. If you use a pressurized tank you need to make note of when you get the bubbles. If you get bubbles only when the engine is idling then it is from an air leak. If at high speed then it is from the pressure drop across the needle valve causing a small amount of the fuel to vaporize. If the latter you can loop the line above or below the needle valve to reduce the chance of the bubble getting to the engine, but usually if it does the engine will only have a quick burp and not.

If you don't use a pressurized tank then I recommend that you do.